Electrical circuits for communication networks



R. E. LUNNEY Aug. 11, 1964 ELECTRICAL CIRCUITS FOR COMMUNICATIONNETWORKS Original Filed July 24, 1957 5 m m m S Skid QEHSx R Q ox n! \t1 E 3&3 $.55 ESE 550m wwmmu -326 h v w m Q Q M BR a D 59% 5.5: .561 R2813 36 w omu on mu r INVENTOR R. E. L UNNE Y ATTORNEY United States Patent3,144,636 ELECTRICAL CERCUETS FGR (It) 2 NETWORKS Raymond E. Lunney,Madison, N.J., assignor to Bell Telephone Laboratories, incorporated,New York, N.Y., a cnrporation of New York Original application July 24,1957, Ser. No. 673,861, now Patent No. 2,972,683, dated Feb. 21, 1961.Divided and this application Nov. 30, 1969, Ser. No. 72,604

Uaims. (Ql. 340-147) This invention relates to electrical circuits and,more specifically, to circuits for the establishment and control of apath through a communication network.

The present application Serial No. 673,861, filed July 24, 1957, issuedFebruary 21, 1961, as Patent No. 2,972,683.

In switching control networks for communication cir cuits, such as intelephone central office switching networks for example, the requiredfunction is the establishment and control of communication paths betweenpairs of a large number of spatially separated subscribers, and varioussystems have been devised to perform this function. One such system isdisclosed in Patent 2,686,837 of S. T. Brewer and E. Bruce, issuedAugust 17, 1954. This system follows the method of marking severalalternate paths between the points to be connected and then selectingone of these paths by a second marking condition at an intermediatepoint.

The communication paths of a system of this kind are composed of anumber of crosspoint devices interconnected in series-parallelcombination in a fan-out configuration. Various devices suitable for usein such crosspoints are known in the art. The gas diode is one suchdevice and a system utilizing gas diodes for crosspoints in acommunication network is disclosed in Patent 2,684,405, issued July 20,1954, to E. Bruce and H. M. Straube. Another switching system in whichtransistors are employed as the crosspoint devices is disclosed inPatent 2,876,285 of B. G. Bjornson and E. Bruce, issued March 3, 1959.

It is common for systems of the type referred to previously to separatethe crosspoint switching network at some intermediate point by a controlcircuit which will be herein designated a junctor, although it has alsobeen referred to as a bisector, or mactor circuit. Utilization of ajunctor intermediate in a cross point switching network introduces anumber of advantages. The margin requirements imposed on the associatedcrosspoint circuitry are thereby rendered less stringent and the fan-outnetwork of interconnected crosspoints is less complex with acorresponding reduction in required equipment. Furthermore, junctorsfacilitate the control function in the switching of the communicationpaths through the crosspoint network. In Patent 2,883,470 of G. E.Jacoby and I. W. Rieke, issued April 21, 1959, a communication networkemploying gas tubes as crosspoints and junctors is disclosed in whichthe junctors function both to establish and to disestablish the pathsthrough the network.

An object of my invention is the provision of an im proved junctorcircuit for a communication network.

A further object of my invention is the provision of a junctor circuitcapable of functioning with simplified control circuitry.

crosspoint switching networks of the type just described commonly have apair of symmetrical circuit configurations on either side of a junctorcircuit. Previously, it has been found necessary to apply a markingsignal to the central junctor circuit as well as to the external markingterminals to cause the disestablishrnent of an existing connection. Inone specific embodiment of my invention, I provide a coincident signalcircuit to control a junctor 'ice flip-flop. The flip-flop of thisspecific embodiment makes use of a pair of transistors normallymaintained in anonconducting condition. Upon the simultaneousapplication of the proper signals the flip-flop is switched to its ONcondition, which provides a low impedance to ground from both networkterminals of the junctor thereby establishing and maintaining acommunication path through the network until a marking signal is appliedat either one of the communication network terminals to request thedisestablishment of the communication path. By means of a feedbackcircuit in the junctor flipfiop, the flip-flop is thereby returned toits high impedance state which in turn disestablishes the path throughthe resistor in the emitter branch of the output transistor of thejunctor flip-flop which, when the marking voltage drives a current abovesome critical value through this output resistor, causes bothtransistors to return to their nonconducting state.

My invention permits the flip-flop to be turned off by the same kind ofexternal marking signal that is applied to initiate the connection. Thispermits considerable simplification of the control circuitry used inconjunction with the switching network and eliminates the need for someof the memory formerly provided in previous junctor circuitry.

It is a feature of this invention that a junctor circuit for use with atransistor communication network be so arranged that the junctor pathmay be switch to its high impedance state upon the application of one ofthe same signals which is used to establish a communication pathinitially.

It is also a feature of my invention that an alternating currenttransmission path be provided between a pair of terminals, in which paththe conduction of alternating current is dependent upon the impedance toground through an associated transistor which is in turn controlled byanother transistor arranged with the first transistor in a bistableelectrical circuit.

These and other features of this invention may be better understood by amore detailed description of the drawing, in which:

FIG. 1 is a schematic representation of a communication networkincluding one specific embodiment of this invention; and

BIG. 2 is a more detailed schematic representation of the portion of thecircuit of FIG. 1 appearing between terminals 3d and 31.

FIG. 1 depicts a skeleton circuit representing a communication networkconnected between a pair of telephone stations. The circuit comprises apair of symmetrical sections on either side of terminals 30 and 31. Eachsection includes outer crosspoint device 1 and inner crosspoint device 2together with associated circuitry to provide for breaking down thecrosspoints upon the application of proper control signals. Betweenterminals 30 and 31 is the junctor circuit which includes an AND gate 6,a junctor flip-flop 5, a transformer 3 and a pair of isolation diodes 4.The presence or absence of a communication path between the twotelephone stations depends upon the state of conduction of thecrosspoints 1 and 2 and the junctor flip-flop 5. Positive potentialsources 8 provide holding voltages for the network which are sufficientin this circuit to maintain the crosspoints 1 and 2 in their lowimpedance state so long as the junctor flipfiop 5 is turned on and thecrosspoints 1 and 2 have been broken down. Negative potential sources 9serve to assist in the breakdown of crosspoint switches 1 upon theapplication of a sufiiciently high positive voltage at the outsideterminal of these crosspoint switches. This high voltage is suppliedfrom positive potential source 7 which is of greater magnitude thanpotential source 8 and which furnishes a marking voltage through switch20 and resistor 14 to initiate the establishment of a communication paththrough the network. Negative potential source furnishes a negativemarking potential through switch 21 to terminal 34 of the particularjunctor which is to be used in establishing and maintaining thecommunication path.

Establishment of the communication path through the network proceeds asfollows:

Marking potentials 7 are applied through switches 20 and resistors 14 tothe external terminals at both ends of the crosspoint network. Workingwith negative sources 9 through resistors 11, the external markingsignals break down crosspoint switches 1. Positive potentials are thenestablished at point 23 between the crosspoint switches 1 and 2 which,acting with negative potential 10 applied to terminal 34 by the closureof switch 21, break down inner crosspoints 2 and drive terminals 30 and31 positive to furnish two of the inputs to the AND gate 6. The negativemarking signal at terminal 34 furnishes the other input to AND gate 6.With positive signals at terminals 30 and 31 and the negative potentialat terminal 34, the AND gate provides a negative output on lead 33 toturn on junctor flip-flop 5 which then provides a low impedance paththrough itself to ground. With a low impedance path established fromterminals 30 and 31 to ground, current can flow through the windings oftransformer 3, thereby providing alternating current transmissionthrough transformer 3 and permitting the communication path to bemaintained by a lower holding voltage. Upon the simultaneous removal ofall marking voltages by the opening of switches 20 and 21, thecommunication path is maintained and the crosspoint switches are held intheir conducting state by holding voltages 8 applied through resistorsand diodes 70.

Disestablishment of an existing communication path through the networkmay be accomplished by the application of either one or both of theexternal marking potentials 7 at the end terminals of the networkthrough switches and resistors 14. In a manner which will be explainedwith reference to FIG. 2, application of either of these potentials 7 inthe absence of the application of negative marking potential 10 causesthe junctor flip-flop 5 to switch to its high impedance state, therebyopening the path through crosspoints 2. Then, upon the removal ofmarking potentials 7, crosspoint switches 1 return to theirnonconducting state and the communication path is completelydisestablished.

It should be here noted that the circuit depicted in FIG. 1 representsbut a single one of the many alternate paths available in a crosspointswitching network. In any operational network, as in those of thepatents hereinbefore cited, multiple connections to alternatecrosspoints or junctors exist at each common point between crosspointsor between crosspoints and junctors. In the interest of clairity andsimplicity these multiple connections are omitted from the diagram ofFIG. 1.

FIG. 2 is a more detailed schematic representation of one specificembodiment of my invention which was shown in simplified form betweenterminals and 31 in FIG. 1. In FIG. 2, terminals 30 and 31 correspond tothe same terminals in FIG. 1. Transformer 3 and diodes 4 correspond totheir counterparts in FIG. 1. The portion of FIG. 2 to the left of thevertical dash line 32, excluding transformer 3 and diodes 4, comprisesthe AND gate 6 of FIG. 1. The portion of FIG. 2 to the right of the dashline 32 corresponds to the junctor flip-flop 5 of FIG. 1. Transistor 40,having emitter 41, base 42 and collector 43, serves as the AND gateamplifier and is normally biased olf by the 44% volt potential sourceand resistors 56, 57, 58 and 60. The application of positive potentialsto both of terminals 30 and 31 forward biases diodes 52 but finds diodes53 and 54 in a reverse biased condition so no change in the state ofconduction of transistor 40 occurs. However, if, while both terminals 36and 31 are positive, the switch 21 is closed to provide a negativepotential from source 10 to terminal 34, the emitter 41 of transistor 40becomes clamped at ground potential by diode 55 and the current throughresistor 59 and diode 54 to negative source 10. Since the base ismaintained at some positive potential between ground and +4 volts, asdetermined by resistors 56 and 57, transistor 40 now conducts and anegative output is delivered through resistor 61 along lead 33 to thebase 46 of transistor 44 in the junctor flip-flop.

Transistor 44, having emitter 45, base 46 and collector 47, is normallybiased off by a positive potential from the +4 /2 volt source deliveredthrough resistor 62. Transistor 44 furthermore controls the state ofconduction of transistor 43 which is also normally biased off by theapplication of a negative potential from the -1 4. volt source throughresistor to the base 50. With the appearance of a negative signal fromtransistor 40 upon lead 33, transistor 44 is turned on and its collector47 is switched near the potential of its emitter 45 which is +3 volts.This turns on transistor 48 through resistor 66, and transistor 48 thenprovides a low impedance path between its collector 51 and its emitter49. Since resistor 67 has a comparatively low value of resistance, thismeans that a low impedance path is provided from terminals 30 and 31through the now forward biased diodes 4, the transistor 48 and resistor67 to ground. As explained before, this permits the crosspoints 1 and 2to be maintained in their low impedance states and establishes thecommunication path through the entire network.

Resistor 67, being connected in the emitter circuit of transistor 48,provides a certain amount of degenerative feedback to that transistor.The resistance of resistor 67 is determined, in conjunction with theresistance in the rest of the crosspoint network and the selection ofthe holding potentials 8 and the positive marking potentials 7, so thattransistor 48 is maintained in its low impedance condition so long asthe crosspoint switches 1 and 2 are held on by the holding potentials 8but will be switched to its high impedance state upon the closure ofeither of switches 20. The current from the 4 /2 volt bias source fortransistor 44 flows through resistors 62, 63 and 64 and the diode 35 andthence through transistor 48 and the resistor 67. If the current throughresistor 67 exceeds a certain critical value the base 46 of transistor44 is driven sutficiently positive to turn off transistor 44, which thenturns off transistor 48, and the junctor flip-flop is returned to itsstate of high impedance. As previously described, this opens thecrosspoints 2 of FIG. 1 to initiate disestablishment of thecommunication path through the network.

As far as the flip-flop circuit is concerned, the collector 51 oftransistor 48 is the output terminal of this circuit. This means thatonce the flip-flop is turned on, it is under the control of the currentwhich is applied to its output terminal. So long as this current remainsbelow some critical value, the flip-flop remains on. When this outputcurrent exceeds some particular value, the feedback provided by resistor67 causes the flip-flop to be turned off. Thus, with control of thejunctor possible from collector 51 of transistor 48 and from therethrough either or both of terminals 30 and 31, it can be seen how it ispossible to achieve disestablishment of an existing communication pathfrom the external terminals of the network alone without the need foradditional memory circuits to keep track of which particular junctor isinvolved in any given communication path or for the provision of specialsignals to be supplied directly to the junctor circuit itself.

If desired the busy or idle state of the junctor circuit can be detectedat a Busy-Idle terminal 71 connected to the collector 47 of transistor44 and to resistor 68, the other side of which is grounded.

The values of voltage shown were those used to provide the properoperation of the depicted specific embodiment of my invention. It shouldbe understood that my invention is not limited to the application of anyparticular voltage source or biasing arrangements.

While switches 20 and 21 are depicted in the drawing as being manuallyoperated switches, it should be emphasized that electronic switchescould readily be employed in place of those shown.

It is to be understood that the above-described circuits are merelyillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. In a communication switching network, a pair of terminals connectedin a path to be established through the network, a transformer having aprimary winding and a secondary winding each having one terminalconnected to a different one of the terminals of said pair of terminals,a first amplifying device, an impedance, means connecting said firstdevice and said impedance in series between ground and the otherterminals of both of said windings, a second amplifying device, acoincidence circuit applying signals to bias said second amplifyingdevice into conduction in response to the simultaneous occurrence ofmarking signals at said path terminals, means connecting an output ofsaid second amplifying device to control the conduction condition ofsaid first amplifying device to be conducting or nonconducting inresponse to similar conditions in said second device, and meansincluding said impedance turning oif said second amplifying device inresponse to the occurrence of only a single marking signal at either ofsaid path terminals when said first amplifying device is conduct- 2. Inthe combination of claim 1, an impedance connected to said output ofsaid second amplifying device and a busy-idle terminal connected acrosssaid impedance.

3. A junctor circuit for a communication switching network comprising alogic circuit and a bistable amplifier circuit, a pair of inputterminals connecting said junctor circuit to adjacent sections of theswitching network, a third input terminal connected to said logiccircuit, means for causing said bistable circuit to assume its lowimpedance state upon the application of signals to all of said inputterminals thereby maintaining a low impedance for alternating currentbetween said pair of input terminals, and feedback means in saidbistable circuit to cause said bistable circuit to return to its stateof high impedance, said feedback means operative upon the application ofa signal at either of said pair of input terminals to present a highimpedance for alternating current between said pair of input terminals.

4. A junctor circuit as in claim 3 in which said logic circuit comprisesan amplifying device having at least positive and negative electrodesand means controlling the impedance of said amplifying device to providea low impedance between said positive and negative terminals only uponthe simultaneous application of signals at all of said input terminalsof said junctor circuit.

5. A junctor circuit as in claim 3 in which said bistable circuitcomprises a pair of amplifying devices having at least positive andnegative electrodes, means connecting the first of said amplifyingdevices to said pair of input terminals, means connecting said logiccircuit to the other of said amplifying devices, and interconnectingmeans between said pair of amplifying devices.

6. A junctor circuit as in claim 5 in which said feedback meanscomprises a resistor connected to the electrode of said first amplifyingdevice opposite the connecting means to said pair of input terminals andsaid interconnecting means.

7. In a communication switching network, a pair of terminals in a pathto be established through the network, a normally high impedance deviceconnected to said pair of terminals, means responsive to the applicationof marking signals at both of said terminals for causing said device toassume a low impedance and enable transmission of alternating currentsignals over said path including said pair of terminals, and meansresponsive to increased conduction through said device on occurrence ofsaid marking signal at either of said terminals when said device is inits low impedance state for causing said device to resume its highimpedance state and prevent transmission over said path.

8. In a communication switching network, the combination set forth inclaim 7 wherein said device comprises a normally nonconductingtransistor having base, emitter, and collector electrodes, one of saidelectrodes being connected to said pair of input terminals.

9. In a communication switching network, the combination set forth inclaim 8 wherein said means responsive to the application of markingsignals at both said terminals includes a second transistor havingemitter, base, and collector electrodes and means connecting one of saidsecond transistor electrodes to said first transistor base electrode.

10. In a communication switching network, the combination set forth inclaim 9 wherein said means responsive to increased conduction throughsaid first transistor further includes means connecting said oneelectrode of said first transistor to said second transistor baseelectrode.

References Cited in the file of this patent UNITED STATES PATENTS2,883,470 Jacoby et al. Apr. 21, 1959

1. IN A COMMUNICATION SWITCHING NETWORK, A PAIR OF TERMINALS CONNECTEDIN A PATH TO BE ESTABLISHED THROUGH THE NETWORK, A TRANSFORMER HAVING APRIMARY WINDING AND A SECONDARY WINDING EACH HAVING ONE TERMINALCONNECTED TO A DIFFERENT ONE OF THE TERMINALS OF SAID PAIR OF TERMINALS,A FIRST AMPLIFYING DEVICE, AN IMPEDANCE, MEANS CONNECTING SAID FIRSTDEVICE AND SAID IMPEDANCE IN SERIES BETWEEN GROUND AND THE OTHERTERMINALS OF BOTH OF SAID WINDINGS, A SECOND AMPLIFYING DEVICE, ACOINCIDENCE CIRCUIT APPLYING SIGNALS TO BIAS SAID SECOND AMPLIFYINGDEVICE INTO CONDUCTION IN RESPONSE TO THE SIMULTANEOUS